This invention relates to a novel bag making machine sealer slitter apparatus. More specifically, this invention relates to a sealer slitter apparatus which transversely seals together and severs two layers of a tubular web of thermoplastic resin film, thereby forming a bag.
Thermoplastic resin bags, such as those used in supermarkets, convenience stores, etc. are produced from a long tubular web of thermoplastic synthetic resin film. The tubular web, which is unrolled from a spool, is intermittently fed into a bag making machine where heat is transversely applied to the web in order to form a weld seal between the two layers of the web. A cutting edge intersects and transversely severs the web adjacent the seal in order to complete formation of a bag.
The sealing and severing apparatus of a bag making machine usually includes a metallic lower stationary seal bar and a corresponding metallic movable upper seal bar. One of the first bag making machines heats one of the seal bars to produce enough heat to melt the thermoplastic web when held between the two seal bars. The cutting edge is positioned adjacent to the movable upper seal bar. When the web is stopped, the cutting edge and the upper seal bar advance together toward the lower seal bar. The web is sealed and severed to produce two separate portions, a leading portion being the bottom of a newly formed bag and a trailing portion being the top of the next bag to be formed.
A problem is often encountered with the above described configuration in that a melted thermoplastic web tends to adhere to the legs of the metallic seal bars. Interruption of the machine cycle is intermittently necessary to clean the seal bars. The advent of a low friction coefficient, heat stable material, such as tetrafluoroethene which is commercially known as Teflon, allowed the web to be sealed without adhering to the seal bars. By coating the surfaces of the seal bars with Teflon, the frequency of interruption of a bag making machine cycle to clean the seal bars was reduced, and overall efficiency was thus increased.
However, production capacity was still limited by difficulties associated with bag cutting operations. In this connection, over time, the cutting edge became blunted, and the bag making machine cycle would be interrupted in order to replace the cutting edge. The integrity of a severed bag edge was also frequently poor due to the elastic property of a thermoplastic web, which is not easily severed from a direction normal to the length of the web.
The effectiveness of the cutting edge was enhanced by heating the cutting edge or knife and thereby allowing both heat and the physical action of the cutting edge to sever the web. However, introduction of heat to the cutting edge encourages adherence of the thermoplastic film or carbon deposits to the cutting edge. A carbon by-product of the thermoplastic film blunts the cutting edge and decreases the sharpness of the severed edge, or kerf, of the thermoplastic film. Unlike the seal bars, the cutting edge is not covered with Teflon because a sharp cutting edge would be difficult to achieve. Therefore, the bag making machine cycle must be interrupted and the cutting edge must be polished in order to remove the carbon by-product. Interruption of the machine cycle again results in a decreased production rate of thermoplastic resin bags.
The difficulties suggested in the preceeding are not intended to be exhaustive but rather are among many which may tend to reduce the effectiveness of and product quality associated with present bag making machine sealing and slitting devices. Other noteworthy problems may also exist; however, those presented above should be sufficient to demonstrate that sealing and slitting devices for bag making machines appearing in the past will admit to worthwhile improvement.